The employment of solutions or dispersions of organic polymers dissolved or dispersed in volatile organic liquids to formulate organic polymer coating compositions and their subsequent application onto various substrates as coatings thereon requires the handling and evaporation of large quantities of organic solvents. Because of the undesirable ecological and environmental problems and problems associated with the exposure of workers in the coatings industry to the organic solvents, alternate coatings methods become a necessity. Consequently there has been a shift to coating techniques where water is substituted for much of the volatile organic solvent or diluent in the coating composition. These are often referred to as "water borne" coating compositions even where water is not the sole dispersing or dissolving vehicle for the organic polymer.
The rise in the use of water borne coatings has introduced problems peculiar to systems containing a mixture of water and organic cosolvent or dispersant. These problems arise from the fact that the evaporation of water is dependent upon the ambient humidity conditions, and the relative rates of evaporation of the organic solvent vis a vis water.
Polymers which were developed for water borne coatings systems were often based on the chemistry of the polymers used in existing solvent systems. Thus alkyds, epoxy esters, and oil modified polyurethanes originally developed for nonaqueous formulations were modified for aqueous formulations by the incorporation of acid moieties.
One of the earliest techniques for the incorporation of acid groups was to maleinize a long oil alkyd or epoxy ester. In this process the alkyd or polyester is reacted at 200.degree.-260.degree. C. for 30-60 minutes with maleic anhydride in the presence of an excess of drying acid. The excess drying acid serves as a reactive chain transfer agent to prevent premature gelation. Accelerators, commonly iodine or sulfur dioxide, were often employed in this thermal polymerization process. The maleic anhydride moiety (now a succinate) is hydrolyzed and neutralized with volatile and/or fixed bases to render the polymer soluble in the water/cosolvent mixture. Enough unsaturation is left in the resin for siccative cross-linking of the final film.
Another early technique was to use dimethylol propionic acid (derived by condensation of propanal with formaldehyde followed by oxidation). This unique acid-diol along with the monoglyceride is reacted with the phthalic anhydride or isophthalic acid to form a polyester/alkyd having free acid groups. The carboxyl moiety of the dimethylol propionic acid is sterically hindered and does not esterify during the polymerization but reacts subsequently with the neutralizing base thus solubilizing the polymer.
Oil modified urethanes are produced by substitution of a diisocyanate for the phthalic anhydride. The urethane reaction is carried out at moderate temperatures (50.degree.-100.degree. C.). These types of polymers form the basis of water borne coatings materials used today in low cost product and consumer finishes.
The next innovation was the development of resins that could be cross-linked by reaction with aminoplasts. These resins contained available hydroxyl functionality as well as an ionizable moiety. The acid containing polymers were used early in electro-coating, and were synthesized by partially esterifying an epoxy resin with a drying fatty acid followed by reaction with trimellitic anhydride. The carboxyl groups incorporated were neutralized in the usual manner. Esterification of the partial epoxy ester with para-aminobenzoic acid in place of trimellitic anhydride yielded early cationic polymers. The cationic polymers are, commonly neutralized with acetic, lactic and other volatile organic acids to attain solubility.
Today the resins are highly specialized for their intended applications. Techniques for the incorporation of the ionic moieties include graft polymerization of carboxylic vinyl monomers to the base epoxy and polyester backbones as well as the development of functionalized telechelic polymers from readily available monomers.
As the coatings industry moves, because of environmental and health regulations, from the more conventional solvent coatings to water borne and high solids systems, there has developed a need for new water borne reactive diluents and reactive cosolvents to serve this emerging technology. At the present time many of these end use needs are being addressed by solvents and reactive materials already in commerce. However, none of these materials were designed for these applications; rather the industry adapted what was available. Today the state of the art has advanced to the point where the real needs are apparent and further improvement in the coating systems requires improved reactive co-solvents and reactive diluents as well as polymers.
For example many of the prior art resin solvents used in water borne coating systems led to degraded polymer properties in the final coatings. Water borne coatings as used herein are coatings in which the principal application medium is water. They may be of two types: (1) dispersions in which organic materials are added at low levels as filming aids, antifreeze additives, defoamers, etc.; (2) solutions in which part of the application medium is an organic solvent. Performance requirements demand that coatings resist the adverse effects of water in the environment and yet in water borne solution coatings technology they are applied from a solution which contains a large fraction of water. This requires that the coatings are not truly water soluble; rather, a number of techniques are employed to maintain solubility throughout application and film formation. The techniques commonly employed are:
(a) Ionic groups are incorporated into the polymer, PA1 (b) A means of crosslinking the film after application is employed, PA1 (c) A cosolvent is used to maintain solubility of the polymer throughout the film forming process. PA1 (i) The cosolvent is miscible with water; line AC is in the one phase region in its entirety. PA1 (ii) The coatings polymer is soluble in the cosolvent; line CB is in the one phase region in its entirety. PA1 (iii) Film formation must take place in such a manner that the phase boundary, A-A'-B, is not crossed. PA1 (iv) Lastly, it is often desired to clean the application equipment with water alone, which means the phase boundary, A-A'-B, must be avoided during cleaning; line EA. PA1 (i) The reactive diluent need not be totally soluble in water, but rather is distributed between the aqueous phase and the polymer phase. PA1 (ii) The reactive diluent is essentially non-volatile and becomes a part of the final coating and consequently the total non-volatiles (total solids) in the composition in the final film, point D, is equal to the sum of the polymer including the crosslinker and the reactive diluent. PA1 (iii) The film forming pathway proceeds from a homogeneous, stable two phase composition to a single phase and therefore the reactive diluent must be compatible (soluble in) with the polymer phase. PA1 (i) The reactive cosolvent is generally miscible with water; line A-C is in the one phase region in its entirety. PA1 (ii) The coatings polymer is soluble in the reactive cosolvent; line CB is in the one phase region in its entirety. PA1 (iii) Film formation can result only from the removal of water (evaporation) and hence the film formation remains in the single phase region thus avoiding the high humidity film defects of the prior art. PA1 (iv) The reactive cosolvent is essentially non-volatile and becomes a part of the final coating and consequently, the total non-volatiles (total solids), in the film composition, point D, is equal to the sum of the polymer including the crosslinker and the reactive cosolvent. PA1 (1) at least one water dispersible, crosslinkable organic polymer free of amide groups; PA1 (2) water; PA1 (3) at least one reactive urea derivative having the generic formula: ##STR1## wherein each of R.sub.1, R.sub.2, R.sub.3 and R.sub.4 is a monovalent radical selected from the class consisting of hydrogen, alkyl groups having 1 to about 10 carbon atoms, hydroxyalkyl groups having 2 to about 4 carbon atoms and one or more hydroxyl groups, and hydroxypolyalkyleneoxy groups having one or more hydroxyl groups and with the provisos that: PA1 (4) A crosslinking amount of a crosslinking agent; and PA1 (5) Optionally, a catalytic amount of a crosslinking catalyst. PA1 Polyester alkyd resins PA1 Carboxylated hydroxyl-containing epoxy fatty acid esters PA1 Carboxylated polyesters PA1 Carboxylated alkyd resins PA1 Carboxylated acrylic interpolymers free of amide groups PA1 Carboxylated vinyl interpolymers, such as styrene/acrylic copolymers.
In the case of solution coatings, it is imperative that during the drying stage, after a particular substrate has been covered with a layer of coating composition that a single phase be maintained until the water and cosolvent components have evaporated away leaving the now insoluble organic polymer deposit. It is also necessary that the cosolvent be miscible with the water and that the organic polymer coatings be soluble in the cosolvent.
The relative volatility of the cosolvent with respect to water involves vapor pressure, molecular weight and the relative humidity during the drying operation. Under high humidity conditions the rate of water evaporation becomes very slow while the evaporation of the volatile organic co-solvent remains relatively constant. Consequently under conditions of high humidity, coatings imperfections develop during film formation which are detrimental to the overall performance of the coatings. These imprefections which form under high humidity conditions are related to the limited solubility of the film forming polymers in the resulting water rich composition. The problem becomes understandable when it is cast in terms of the phase chemistry of a water borne coating.
A simplified phase diagram of a hypothetical water borne system is illustrated in FIG. 1. FIG. 1 is a phase diagram representing water as A, the cosolvent as C and the film forming polymer as B. The area of a single phase, 2, is separated from the area of two phases, 4, by the curved line A-A'-B. The point E represents the application composition of the water borne system. In the case of a volatile cosolvent under ideal conditions (low humidity) the straight line E-B represents the changing composition during film formation. Under conditions of high humidity the dotted line E-S-B illustrates that the film formation pathway has taken an excursion into the two phase region which causes premature coagulation of the film forming polymer and results in coating imperfections.
The points illustrated by FIG. 1 are:
In real systems the phase diagrams are more complicated than illustrated. The shape of the two phase region is more irregular and is not as predictable as indicated. The model does, however, provide a framework for understanding solubility relationships and provides a common basis for understanding the objectives of the present invention.
It is an object of this invention to provide non-volatile, reactive organic materials which are also solvents or diluents suitable for the preparation of water borne high solids coating compositions.
It is another object to provide reactive solvents/reactive diluents which serve to provide high solids coating compositions in the form of either solutions or aqueous dispersions.
It is still another object to provide reactive solvents/reactive diluents which will not degrade the polymers used in the coatings compositions or the properties of the finished coating.
Other objects will become apparent to those skilled in the art upon a further reading of the specifications.
Water borne high solids coatings avoid many of the problems referred to earlier. Again the advantage of water borne high solids coatings compositions can be illustrated by use of phase diagrams. Since water borne high solids coatings may be either of the dispersion type or the solution type, two different phase diagrams will be required to understand the phase relationships. FIG. 2 is a simplified, hypothetical phase diagram of a water borne high solids dispersion coating composition, representing water as A, the reactive diluent as C, and the polymer including the crosslinker as B. The area of single phase, 2, is separated from the area of two phases, 4, by the curved line A-A'-B. The point E represents the application composition of the water borne high solids dispersion coating. The straight line E-D represents the changing composition during film formation; the dashed line E-B is the film forming pathway that the water borne coating of the prior art would take.
The salient features of the phase diagram are:
FIG. 3 is a phase diagram of a water borne high solids solution coatings composition representing water as A, the reactive cosolvent as C, and the polymer including the crosslinking agent as B. The area of single phase, 2, is separated from the area of two phases, 4, by the curved line A-A'-B. The point E represents the application composition of the water borne high solids solution coating. The straight line E-D represents the changing compositions during the film formation process.
The notable features of FIG. 3 are:
In both cases the reactive diluent and/or reactive cosolvent becomes a part of the final coating and as such must contribute to, not detract from, the overall coatings performance. The reaction product of polymer, reactive diluent and/or reactive cosolvent with the crosslinking agent must have suitable properties such as toughness, adhesion, impact resistance, abrasion resistance, scratch resistance, resistance to solvents, chemicals, acids, bases, and have good color, gloss and stability as is required according to the end use application. This is well understood by those skilled in the art.